Page 301 - Mechanism and Theory in Organic Chemistry

Collins, on the other hand, has proposed that the memory effect can be
explained, at least for some systems, by "counter-ion control" : The leaving group
stays near the site of ionization and thereby influences the future steric course of
the reaction.56

6.2 CARBONIUM IONS 57-60
In this section we shall discuss carbocations in which at least one carbo~.tlumgh-
a three-center - bond (see Section 5.3) is coordinated to four~fivg~om~.
By
..7.p-.-..-
_C_-
Olah's terminoloxyY the5e-a~ cccarb~nium ions7'__as opposed to tricoordinated
c c
carbenium ions."61 By older terminology the more highly coordinated carboca-
tions are called "nonclassical carbonium ions" to differentiate them from the
tricoordinated "classical carbonium ions."
Homoallylic Carbonium Ions62
In 1946 Shoppee noted that the reaction of 3-P-cholesteryl chloride with acetate
ion proceeds entirely with retention of configuration (Equation 6.29). Substi-
tutions on the analogous saturated compound proceed with the expected

66 C. J. Collins, I. T. Glover, M. D. Eckart, V. F. Raaen, B. M. Benjamin, and B. S. Benjaminov,
J. Amer. Chem. Soc., 94, 899 (1972).
51 For a review on the general subject, see P. D. Bartlett, Nonclassical Ions, W. A. Benjamin, Menlo
Park, Calif., 1965.
58 For reviews of homoallylic and small-ring participation, see: (a) R. Breslow, in Molecular Re-
arrangements, P. Mayo, Ed., Wiley-Interscience, New York, 1963, Vol. 1, p. 233; (b) M. Hanack and
H.-J. Schneider, Angew. Chem., Int. Ed., 6, 666 (1967); (c) R. R. Story and B. C. Clark, in Carbonium
Ions, G. A. Olah and P. v. R. Schleyer, Eds., Wiley-Interscience, New York, 1972, Vol. 111, p. 1007;
(d) K. B. Wiberg, B. A. Hess, and A. J. Ashe, in Carbonium Ions, Olah and Schleyer, Eds., Vol. 111,
p. 1295; (e) H. G. Rickey, in Carbonium Ions, Olah and Schleyer, Eds., Vol. 111, p. 1201.
" For reviews of bicyclic carbonium ions, see: (a) J. A. Berson in Molecular Rearrangements, P. Mayo,
Ed., N.Y., Vol. I, p. 111: (b) G. D. Sargent, Quart. Rev. (London), 20, p. 301 (1966); (c) G. D.
Sargent, in Carbonium Ions, Olah and Schleyer, Eds., Vol. 111, p. 1099.
ao For an opposing view, see (a) note 25(a), p. 277; (b) H. C. Brown, Chm. C3 Eng. News, 45,
Feb. 13, p. 87 (1967); (c) H. C. Brown, Accts. Chem. Res., 6, 377 (1973); (d) E. N. Peters and H. C.
Brown, J. Amer. Chem. Soc., 96, 263, 265 (1974); (e) H. C. Brown, M. Ravindranathan, and E. N.
Peters, J. Amer. Chem. Soc., 96, 7351 (1974).
G. A. Olah, J. Amer. Chem. Soc., 94, 808 (1972).
a2 See note 58.